Various techniques have been used in the past to achieve noninvasive imaging of blood flow using ultrasound. Recent developments in Doppler echocardiography are an example. Although the present invention is applicable to other uses, it will be described below in connection with its applicability to Doppler ultrasound blood flow imaging.
A typical ultrasound blood flow imaging system includes an ultrasonic transmit-receive transducer for transmitting ultrasonic pulses into a region of the body under diagnosis and for receiving echo signals of the transmitted ultrasound waves reflected by blood flowing in the area being scanned. One type of transducer is in the form of a probe containing a curved linear array of piezoelectric elements that insonify a sector shaped area of the body. A typical diagnosis with ultrasound includes scanning the patient with the ultrasound probe to measure blood flow rate in an artery, a vein, or in the heart. A signal processing system processes the received echo signals for measuring the Doppler shift frequency of the echo signals to thereby calculate the velocity of the blood flow, and the result of the velocity distribution measurement is displayed as a Doppler blood flow image. Techniques have been conventionally used for displaying the Doppler shift as a black and white image displaying velocity (B-mode gray scale display of echo amplitudes); in more recent years, color imaging techniques have been developed for displaying the two dimensional velocity distribution of blood flow in the area under diagnosis.
In order to estimate the Doppler shifts: of the echoes received from the blood cells, an ultrasonic imaging system commonly transmits several (e.g. 4-16) pulses at one angle in the region under diagnosis and then detects the variations in the phase of the echoes from pulse to pulse.
Echo signal components reflected from stationary targets are removed, while components reflected from very slowly moving (near-stationary) targets such as moving tissue, are, for the most part, only partially removed. These stationary and near-stationary tissue motion signals are referred to as "clutter." Their complete removal is desirable since their relative amplitude is typically orders of magnitude greater than the Doppler-shifted signals resulting from blood flow.
A stationary cancelling filter (also called a moving target indication filter or MTI filter) is used to eliminate signals caused by stationary objects, and to partially eliminate signals caused by near-stationary objects. In a typical MTI filter, echo signals from consecutive sound receptions are subtracted. The subtraction steps eliminate the signal due to stationary tissue and partially eliminate signals due to near-stationary tissue. The MTI filter output is then processed by a velocity estimator to extract the Doppler frequency information, which is converted to velocity data signals suitable for display in color or on a B-mode gray scale display of echo amplitudes.
Ideally, the extracted Doppler frequency information contains only those Doppler signal components representing blood flow. In practice, however, this information also includes components representing moving tissue, which are not removed by the MTI filter. Such tissue motion may be due to breathing, heartbeat, probe motion, or the like. In general, the relative amplitude of the signals due to tissue motion is orders of magnitude greater than the signals representing blood flow. The typical MTI filter will not cancel all of these signals because they are not completely stationary. When sufficient tissue motion occurs, a color smear appears over the velocity display image, making it difficult to discriminate between actual blood flow and tissue movement artifacts. This is so because the blood flow information which is sought is obscured by the color induced by tissue motion.
The present invention is concerned with improving the accuracy of blood flow estimation of a Doppler color flow imaging system. This objective is achieved by providing a technique for estimating tissue velocity, and for cancelling this velocity component from the Doppler frequency signals to produce a blood flow signal compensated for the errors produced by tissue motion.